Sir Alex Ferguson has demanded that Wayne Rooney brush up on his penalty technique.

After his failure against Arsenal last weekend, the Manchester United striker almost blew it again on Wednesday night as he slipped just as he was about to let fly from the spot in Braga. Thankfully for Rooney, his shot still went in.

Speaking ahead of tonight's visit to Aston Villa, Ferguson said: "Wayne has to accept he must have a better strike rate at penalties. He is capable of it. The one thing about Rooney is it doesn't matter if it's the World Cup final or the first round of the cup against an amateur team, he will want to score and if he misses it, he doesn't let it affect him."

Ferguson has predicted both Rooney and Javier Hernandez will eventually reach the 20-goal mark this season, although Robin van Persie seems certain to sail past that figure.

The Dutch sharp-shooter has enjoyed a stellar start to his United career and took his goal tally to 11 in 14 games in midweek.

It is a strike rate even better than that of fellow countryman Ruud van Nistelrooy, who took 19 games to reach the same figure following his transfer from PSV Eindhoven in 2002. The comparison between the pair is an obvious one to make.

But Ferguson dismisses it, and added: "Robin is a very different player to Van Nistelrooy. Ruud was a penalty box player. Robin has a different understanding of the game.

"I never thought we'd get him. There was always the feeling that Arsenal would find a way to get him to stay. But once he made his mind to leave we firmed up our interest because he is one of the best in the business."

Whilst Van Persie's contribution cannot be understated, United have had a staggering 15 different goalscorers this season, including Jonny Evans and Patrice Evra, who have two each.

"Fifteen goalscorers is very healthy and satisfying," said Ferguson. "It is getting ridiculous though - Evra has virtually doubled his tally for his career. We will take them from anyone. I don't care who gets them."